The present invention relates generally to systems for accessing various remote instrumentalities, and in particular, to a circuit for selectively activating a unit capable of interfacing with the instrumentality responsive to detected tones.
Recently, there has been significant interest in the development of systems for interrogating various remote instrumentalities to obtain desired information without requiring personnel to travel to the remote location at which these instrumentalities are stationed. One general class of remote interrogation involves the detection of alarms indictating that the instrumentality in question has undergone a certain change in state. This change in state may signify any of a number of alarm conditions such as a fire, an unauthorized entry, the departure from specified norms, the measurement of a specified parameter, etc. Another general class of remote interrogation involves the accessing of meters and other information gathering devices. This may include the accessing of electric meters, gas meters and others, as well as devices for monitoring any of a number of parameters including flow rates, temperatures, pressures, etc.
Generally, such interrogations are accomplished by transmitting a tone or group of tones to the instrumentality to be accessed, so as to cause the instrumentality to read the parameter or parameters which it monitors, and to transmit this information to the source of the interrogation signal. Ordinarily, these operations occur over telephone lines, since this eliminates the need to run separate lines between the source of the interrogation signal and the remote instrumentality. However, this has the disadvantage of subjecting such operations to the regulations imposed upon, and imposed by, the various telephone companies involved. One such regulation, with which the present invention primarily concerns itself, relates to the use of current from telephone lines to operate associated equipment.
Various regulations place specific limits on the amount of power which can be drawn from a telephone line, so as to avoid an unacceptable interruption in telephone service, and so as to enable the fault detection circuitry which is conventionally used by many telephone companies to operate correctly. When a telephone line is placed in service (the so-called "off-hook" condition), sufficient amounts of current may generally be drawn from the telephone line to operate most available remote monitoring devices. However, this requires continued occupation of the accessed telephone line during these periods, which is often unacceptable, and at times impossible, since the transmission of speech and data communications must not be affected by such ancillary equipment. When the telephone line is not in service (the "on-hook" condition), prevailing regulations generally severely limit the amount of current which may be drawn from the telephone line, often to only several microamps, which is generally insufficient to operate most presently available remote monitoring devices.
In many cases, the remote monitoring device forms part of a requested service, or a service to which a customer subscribes. In providing services of this type, the customer receives the benefit of the service and expects the service to be performed. Accordingly, the company which provides the service is generally able to rather freely access the customer's power lines, which avoids the need to deal with the above-discussed constraints.
However, in connection with certain types of meter reading equipment, the remote monitoring device is operated for the convenience of a utility or some other third party, and not for the convenience of the customer in possession of the premises at which the remote instrumentality is stationed. Since these operations are for the convenience of a third party, and not the customer, federal regulation prohibits such systems from drawing power from the customer's premises. Accordingly, the only available source of power for operating the remote monitoring device is often the telephone lines which enable communication between the interrogation system and the remote instrumentality being interrogated, subjecting the third party to the above-discussed constraints. Moreover, since access to the customer's telephone lines is then rather limited (generally to off-peak hours), and since ringing of the customer's telephone is to be avoided, the normally on-hook monitoring device will generally not receive sufficient current to operate as required.
It therefore has become desirable to develop a means for operating a system for the remote interrogation of an instrumentality which is capable of drawing necessary operating current from the telephone line over which such communications take place, eliminating the need to draw current from the premises at which the remote instrumentality is stationed, yet which is capable of doing so without drawing more current than is permitted by present regulations in this regard.